Roasting coffee is a process bursting with a huge variety of chemical reactions and physical changes, transforming green coffee into the aromatic roasted coffee we’re all familiar with. These changes happen as a function of thermodynamics, which in chemical terms is the study of heat, work, and the chemical reactions and physical changes which result from those influences. “Each choice a roaster makes with time and temperature create the profiles of roasting, because each decision impacts the rate of moisture loss, external bean temperature versus internal bean temperature, and the coffee's microchemistry,” explains Ildi Revi, M.Ad.Ed., Ally Coffee’s Director of Performance. So then, what exactly are those chemical reactions that help determine roasted coffee profiles? And, if those reactions are determined by heat application decisions made by roasters, how can you begin to influence those profiles more intentionally?
This blog is written to guide you through some of the major reactions that your green coffee undergoes, and to provide a base-level understanding of how they interact to produce the final roasted product that you share with your customers.
The Maillard Reaction
The Maillard Reaction is one of the most commonly talked about chemical reactions in coffee roasting. This is a non-enzymatic browning reaction caused by the interaction of simple sugars and amino acids, particularly in the presence of heat. The interaction breaks apart these simple sugars and amino acids in order to form a wide range of new flavor and aromatic compounds.
The Maillard Reaction typically proceeds rapidly at temperatures ranging 280–330°F (140–165°C), but can begin at lower temperatures and will continue into first crack. As temperatures reach 150°C (302°F) aromatic compounds are produced such as furans, which impart a sweet, caramel-like note to the coffee. In the late stages of the Maillard Reaction, generally as you approach 320°F, melanoidins are formed as soon as you start to see the browning of the green coffee. These melanoidins contribute to the color of roasted coffees, as well as the weight and texture of the resulting brew.
At The Roaster
The amount of heat you apply and rate at which you move through the Maillard Reaction when roasting coffee will have a significant effect on the final product. In general, the faster you move through this phase the greater the acidity and sweetness in the cup; the slower you roast, the more rounded the acidity, sweetness, and body.
A Few Aromatic Compounds Created During Roasting| COMPOUND | TYPE | AROMA / FLAVOR QUALITY |
| Furaneol | Furan | Sweet, caramel |
| Acetaldehyde | Aldehyde | Pungent, fruity |
| Methylpropanal | Aldehyde | Floral, spicy |
| 2,3-pentanedione | Diketone | Buttery |
Caramelization
Caramelization is another non-enzymatic browning reaction which contributes to the color, aroma, and flavor of roasted coffee in a substantial way. Caramelization is the browning of sugars, both simple and complex, via the influence of heat. The chemical results of caramelization echo those of the Maillard Reaction, producing greater concentrations of melanoidins, along with flavor and aromatic compounds like ketones, esters, and aldehydes.
The temperature at which caramelization begins depends on the rate of the temperature increase. Sucrose, one of the primary sugars in green coffee, might begin caramelizing at 320°F/160°C if heat is rising a few degrees per minute. However, a faster temperature increase may push sucrose caramelization to start above 356°F/180°C. Different sugars will also begin caramelizing at different temperatures, and caramelization will also cause more complex sugars like sucrose to break into simpler sugars like fructose and glucose, adding to the complexity of caramelization in coffee roasting.
Ultimately, caramelization’s greatest contribution to roasted coffee comes by way of the sugar-browning aromas imparted on the coffee. These result from sugar browning reactions, and are composed of aldehydes, ketones, sugar carbonyl compounds, and pyrazine compounds, providing aromas that can span herbal, malty, floral, spicy, fruity, and more.
At The Roaster
Balancing the caramelization process in your roast profile can be difficult. Too little caramelization will result in roasted coffee that is lacking in both aromatics and body, while extended caramelization will break down important compounds like the organic acids which contribute to many of the fruit-like notes found in coffee. Remember that caramelization will be impacted not only by your final temperature, but also your total roast time and the speed at which you increase the temperature of the batch.
Strecker Degradation
Strecker Degradation is a less-discussed reaction in coffee roasting, but one that has a significant impact on the final roasted product. The reaction itself is the conversion of an amino acid into an aldehyde, ammonia, and CO2 in the presence of an oxidizing agent.
Strecker Degradation has two major influences on roasted coffee. First, the aldehydes produced during the reaction are critical aromatic compounds in roasted coffee spanning a broad spectrum of aromas including fruity, floral, grassy, nutty, and more; for example, the Strecker-related aldehydes 2-methylpropanal and 3-methylbutanal are responsible for malt flavor notes in coffee. Second, the CO2 produced in the reaction is responsible for second crack in the roaster for people who roast on the darker side.
At The Roaster
The impact of Strecker Degradation on your coffee, and the impact of your roasting approach on Strecker Degradation, are complicated threads to unravel. A faster roast tends to produce higher concentrations of Strecker-related aldehydes early in the roasting process. However, those aldehydes tend to break down at higher temperatures, lessening their aromatic additions. We also know that the Maillard Reaction creates a common oxidant for Strecker Degradation, which implies that extending the Maillard phase of the roast produces greater opportunities for Strecker Degradation to occur in the later stages of roasting. This can be valuable to consider as you develop a roasting profile; by extending the Maillard phase, and thereby creating more opportunities for Strecker Degradation to occur, you can increase your chances of having high concentrations of those critical aromatic compounds in your roasted coffee.
Pyrolysis
Pyrolysis is the decomposition of materials at high temperatures in an inert, or chemically inactive, environment. Sugar caramelization is the most significant pyrolytic change that occurs during coffee roasting, but pyrolysis can also occur when roasting beyond first crack and into second crack. This type of pyrolysis results in darker coloration of the coffee, higher concentrations of melanoidins, and coffee that tastes “roasty” or, in more extreme cases, “charred”.
At The Roaster
Pyrolysis typically occurs in the later stages of roasting, and its effect on the cup profile becomes greater as you increase your intended end temperature. Roasting green coffee with extended caramelization that moves into greater pyrolysis will result in coffees with a heavier body and rounded mouthfeel compared to a lighter roasted batch of the same green coffee. These coffees will also have a lower perceived acidity, fewer and less intense fruit notes, and will display more “roasted” flavor notes in the cup.
Changes in Organic Acids
Organic acids are major contributors to the characteristics and quality of a final cup of coffee. Though the number of chemical compounds found in coffee are vast, this section will only look at a few which have a significant and distinct sensory impact: the organic acids citric, malic, quinic, and acetic, as well as chlorogenic acid (CGA).
When green coffee is roasted, some acids decrease from their original concentration level while others are formed during roasting and increase in concentration. Citric and malic acids, both contributors to fruity cup characteristics, are formed during seed development in the coffee cherry and decrease over the course of roasting beginning around 350°F/176°C and 375°F/190°C respectively. Conversely, acetic acid is created during the roasting process as some sugars like sucrose are broken down. Acetic acid may rise to triple its original concentration during roasting, continuing to increase up to 464°F/240°C.
Chlorogenic acid is not an organic acid itself, but its relation to organic acids in your roasted coffee and the potential sensory quality of your final product is significant. CGA is the ester of caffeic acid and quinic acid, both of which contribute to bitterness in coffee. As CGA degrades through the roasting process, quinic and caffeic acids can both be byproducts of the reaction (along with a variety of other cascading reactions), increasing their concentration as CGA decreases and resulting in a cup with more bitter notes.
The Four Main Organic Acids in Coffee| COMPOUND | SENSORY IMPACT | IMPACT OF ROASTING |
| Citric Acid | Sour flavor, lemon- or lime-like. | Concentration decreases during roasting starting around 350°F/176°C. |
| Malic Acid | Tart flavor, green apple-like | Concentration decreases during roasting starting around 375°F/190°C. |
| Quinic Acid | Bitter flavor, cranberry-like | Concentration roughly doubles during roasting. |
| Acetic Acid | Main acid in vinegar, vinegar- or wine-like | Concentration continues to increase throughout roasting, up to 464°F/240°C. |
At The Roaster
Controlling organic acid concentrations in your roast can be difficult even if you know the ways in which those acids change in relation to temperature. Each coffee, depending on factors like variety, origin, and production processes, will begin with different amounts of various organic acids and other compounds before you start roasting. See the table above for some guidelines on changes in organic acids that will occur during your roasting process.
The Role of the Roaster
In many ways, the role of the coffee roaster is to balance, fine tune, and repeatably execute a number of interrelated and complex chemical reactions happening in their roasting machine. While some reactions are independent of others, some rely on the products of reactions before them in order to reach their fullest potential. This means that changing any variable in your roast—charge temperature, gas settings, or timing of your temperature changes, to name a few—will ultimately have an impact on your final roasted coffee.
Understanding these chemical reactions can be a benefit to having greater control over your process, but understanding that every coffee you roast is unique is important as well! Approach each coffee with an open mind, experiment to see how changing a roast variable changes the cup, and always keep notes to help use your new chemical reactions knowledge to understand your coffee more profoundly.
Learn More
Ready to learn more about roasting? Check out The Essential Coffee Roasting Course featuring Rob Hoos and The Comprehensive Coffee Sample Roasting Roasting Course featuring Mark Michaelson!